Plasma torches are used to treat matter (solid, liquid or gas) at very high temperatures in a controlled-reactivity atmosphere. Plasma torches are traditionally used in particular in welding, marking, thermal spray and waste treatment.
Plasma is a gas in ionized state, traditionally considered to be a fourth state of matter. To obtain the ionization of a gas at atmospheric pressure, plasma torches are used. These contribute the energy needed for ionization of the gas using an electromagnetic wave (radio frequency or microwave) or an electric arc. We are only considering arc torches here, which constitute the only technology making it possible to reach significant operating powers.
Arc torches are classified into two categories: sprayed-arc torches and transferred-arc torches. In the case of sprayed-arc torches, the two electrodes making it possible to establish the arc are contained in the torch and the arc is therefore confined therein. The plasma plume created by the passage of a gas in the arc is ejected outside the torch. In the case of transferred-arc torches, the torch comprises only one electrode and the arc is established between the torch and another material serving as counter-electrode. Examples of sprayed-arc torches and transferred-arc torches are described in application EP-A-706308.
Two transferred-arc torches can be used together in order to maintain an arc between them, one serving as cathode and the other as anode. This device is known under the name “twin torches”.
One example of twin torches is described in application EP-A-1281296.
Regardless of the arc torch technology implemented, the main problem remains the short lifespan of the electrodes.
For several years, a number of research projects have essentially related to improving the lifespan of plasma torch electrodes through the choice of material for the electrodes. These electrodes are classified into two categories: so-called “hot” electrodes, made in a refractory material with a high boiling or sublimation point such as tungsten and zirconium, and so-called “cold” electrodes, made in a material with a low boiling point and strong heat conductivity such as copper. Regardless of the type of material used, the electrode is subject to wear via erosion.
Different technological solutions have been developed to decrease the speed of wear of the electrodes: doping of the tungsten with thorium, machining of the end of the electrode, etc. The need to cool the electrode itself through internal water circulation quickly appeared and had the main result of making the architecture of the torches more complex, the presence of two or even three separate cooling circuits hardly being compatible with systems of limited size like thermal plasma torches. Moreover, maintenance and replacement operations for the electrodes are made difficult by the need to first disconnect the connections of the cooling circuits.
It is also known in a field distant from that of plasma torches, i.e. aluminum electrolysis or the steel industry, to use consumable electrodes, existing in the form of a simple solid graphite cylinder. However, the only possible applications of these electrodes are in a reducing gaseous atmosphere, as in an oxidizing atmosphere, the combustion of the graphite would lead to rapid erosion of these.
The aim of the present invention is to provide a transferred-arc plasma torch having the same properties for use as cooled-electrode plasma torches but without having the drawbacks thereof, in particular in terms of bulk and the complexity of assembly and maintenance.